1. Field of the Invention
The present invention relates to a wavelength-variable optical semiconductor device used in optical communication systems. Specifically, the present invention relates to an optical semiconductor device wherein the increase of spectral line width due to the reflected light being reflected at the reflecting point present in the device and returning to the semiconductor laser can be inhibited.
2. Background Art
In the long distance communication system using relay by an optical amplifier, DWDM (Dense Wavelength Division Multiplexing) is used for increasing the transmission volume for one optical fiber. In this system, optical signals of about 80 different wavelengths are multiplexed in one fiber. At present, the development of wavelength-variable lasers that can oscillate at optional wavelengths from the used wavelength band has progressed, which has become the mainstream of the light source for long-distance optical transceivers.
As a modem method, an IM-DD (Intensity Modulation-Direct Detection) system has been used in systems having the signal speed of up to 10 Gbit/s. In recently penetrating 40 Gbit/s system, phase modulation and differential detection methods are used. In the digital coherent system adopted in next-generation 100 Gbit/s systems, phase modulation systems are used. In the signal receiving side, a coherent detection system wherein local light and signal light are mixed to detect the intensity and phase information are used.
In the conventional IM-DD system, since no phase information of the light is used, it is enough if the light source oscillates at a single wavelength, the phase noise causes no problems. However, in the digital coherent system, the phase noise of the signal light source and the local light source causes the deterioration of signal qualities. Although a spectrum line width is used as the indicator showing the size of the phase noise of the light source, it is required to narrow the spectrum line width for lowering the phase noise.
As a method for realizing the wavelength-variable light source, an optical semiconductor device wherein a plurality of semiconductor lasers and optical amplifying sections are accumulated has been reported. In this method, any one of a plurality of semiconductor lasers arrayed in parallel is made to flash, and the output light thereof is output from a waveguide via a wave coupling section. By amplifying the output light in the optical amplifying section, light having a desired wavelength is output at a desired optical power.
The spectrum line width Vo has generally the relationship shown in the following numerical expression 1.V0∝(k LDFB)−2(LDBF)−1(1+α2)  [Expression 1]For realizing a narrow line width, it is desired to lengthen the laser length LDFB. Also in the above described optical semiconductor device wherein the semiconductor lasers and the optical amplifying sections are accumulated, it has been reported that the low line width of 1 MHz or below is realized by lengthening the laser length. However, there are causes to deteriorate the spectrum line width. In the case wherein a reflectivity on the front end surface is limited, the light reflected by the front end surface is again amplified by the optical amplifying section, and the reflected light returns to the semiconductor laser and causes adverse effects.
When a reflectivity on the front end surface is made to be R0, the spectrum line width Δv when fed back is represented the following Numerical Expression 2 (for example, refer to IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 3, May/June 2009, pp. 514-520).
                                          Δ            ⁢                                                  ⁢            v                                v            0                          =                  1                                    [                              1                +                                  C                  ⁢                                                                          ⁢                  sin                  ⁢                                      {                                                                  ω                        ⁢                                                                                                  ⁢                        τ                                            +                                              ∅                        c                                            +                                              arctan                        ⁡                                                  (                          α                          )                                                                                      }                                                              ]                        2                                              [                  Expression          ⁢                                          ⁢          2                ]            Where, there is the relationship of the following numerical expressions 3 and 4.
                    C        =                                            R              ext                                ⁢                      L            ext                    ⁢                                    P              DFB                                                      P                av                            ⁢                              L                DFB                                              ⁢                                    K              Z                                ⁢                                    1              +                              α                2                                                                        [                  Expression          ⁢                                          ⁢          3                ]                                          R          ext                =                                            R              0                        ⁡                          (                                                P                  SOA                                                  P                  DFB                                            )                                2                                    [                  Expression          ⁢                                          ⁢          4                ]            Where, v0 represents the line width when C=0, i.e. R=0; τ represents the time required for one round trip of the oscillator exterior to LD.
From the Numerical expression 2, when there is the feedback due to reflections, the spectra line width changes periodically, and becomes maximum when it is nearlyC sin {ωτ+φc+ arctan(α)}=−1When change in the angular frequency for the current value applied to the laser is approximated as in the following Numerical expression 5, the line width changes periodically by the current values applied to the semiconductor laser as shown in FIG. 3 (a) in IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 3, May/June 2009, pp. 514-520).ω−ω0=αI2DFB+bIDFB  [Expression 5]